This invention relates to a device for storing electrical energy which typically is denoted as a secondary electrochemical cell of the general type described in the Shimotaki et al, U.S. Pat. No. 3,488,221 issued Jan. 6, 1970 assigned to a predecessor in interest of the assignee of this application. Typical secondary cells have long shelf lives, and may be completely and repeatedly charged and discharged at either rapid or slow rates and can produce extremely high currents for short periods of time. Secondary cells are versatile and may be used as constant voltage sources over long periods of time or as sources for large currents for short period of time. Secondary cells of this general type have uses in space and other remote areas.
In prior developed high-temperature secondary electrochemical cells, the positive electrode generally has been formed with chalcogens such as sulfur, oxygen, selenium or tellerium, as well as their transition metal chalconides, and have included the sulfides of iron, cobalt, nichel and copper.
In high temperature cells, current flow between electrodes often is transmitted by molten electrolytic salt. Particularly useful salts include compositions of the alkali metal halides and/or the alkaline earth metal halides ordinarily incorporating a salt of the negative electrode-active metal, such as lithium, see column 2 of the Shimotaki et al, U.S. Pat. No. 3,488,221. One problem with many of the electrolytes available is the limited dynamic range, which is the ratio of alkali or alkaline metal earth ions for which the electrolyte will remain liquid, at a specific temperature, to avoid the electrolyte from solidifying in the electrodes as the concentration of positive ions changes during cell operation. The larger the dynamic range the more useful the electrolyte since electrolyte solidification reduces electrode efficiency.
Alkali metals such as lithium, sodium, potassium or alkaline earth metals including calcium, magnesium and others along with alloys of these materials have been used as negative electrode active materials. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-aluminum-silicon, lithium-magnesium as well as many others have been used to improve retention of the electrode active material at the high operating temperatures of these secondary electrochemical cells.
A preferred cell along with a method of making an anode was disclosed in U.S. Pat. No. 4,386,019 issued May 31, 1983 to Kaun et al, assigned to the assignee of this invention, the entire disclosure of which is incorporated herein by reference. In the U.S. Pat. No. 4,386,019, a method of fabricating an anode was disclosed, but that method is applicable to fabricating both electrodes of the present invention. In my prior U.S. Pat. No. 4,446,212 issued May 1, 1984, assigned to the assignee of this invention, the entire disclosure of which is incorporated by reference, it was stated that a disadvantage of previous cells incorporating a lithium-aluminum electrode was the reduction in cell capacity during prolonged operation, and the patent disclosed a cell including a basic lithium-aluminum negative electrode with an iron sulfide (FeS or Fe.sub.2 S)positive electrode along with an electrolyte blended of lithium chloride and potassium chloride. In this positive electrode, the typical loading density was 1.5 Ah/cm.sup.3, a fairly typical density. The invention there disclosed included the addition of an aluminum-iron alloy, and/or graphitized carbon and/or magnesium oxide to the negative electrode in order to reduce the declining capacity due to repeated discharge.
In the design and consideration of iron sulfide secondary cells, it has long been known that the higher voltage (1.75 avg. V) reaction of FeS.sub.2 +2 Li.sup.+ +2e.sup.- .fwdarw.Li.sub.2 FeS.sub.2 is endothermic. It has also been well known that the lower voltage (1.33 avg. V) reaction of Li.sub.2 FeS.sub.2 +2Li.sup.2 +2e.sup.- .fwdarw.2Li.sub.2 S+Fe is exothermic, whereby the net positive electrode reaction is exothermic. The typical iron sulfide cell has operated using both the higher voltage reaction, hereafter the "upper plateau" and the lower voltage reaction, hereafter the "lower plateau", so that the conventional iron sulfide electrode was a two plateau (t.p.) electrode. It has long been desirable to provide an electrochemical cell which is endothermic at the positive electrode but until the present invention that has not been possible.
This invention provides a secondary cell which has substantially uniform discharge capacity over extended charge-discharge cycles and operates entirely on the upper plateau reaction, providing a endothermic positive electrode reaction. The vastly improved discharge capacity is due, it is believed, to a combination of a new electrolyte having a low melting point and improved dynamic range, a new higher loading density FeS.sub.2 positive electrode and a cell design that is negative electrode limited which ensures that the positive electrode is limited to its upper plateau reaction. This invention includes a higher density positive electrode for a secondary electrochemical cell, a low melting point electrolyte having an expanded dynamic range, a molybdenum cladded housing and a method of isothermally operating a secondary electrochemical cell.